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Systems for olfaction, touch, vision, taste and hearing are all types of sensory systems which animals can possess. These sensory systems are part of the peripheral nervous system and are dependent on sensory receptors. Of course, more primitive animals will lack sophisticated systems or even sometimes, one of the systems completely.

Within the nervous system, animals possess sensory receptors which can be either complex organs or fairly simple neurons. Basically, these receptors respond to incoming stimuli by converting this stimulus into membrane potential. A change in the membrane potential from a resting state causes a neuron to become excited and thus, send out an electrical signal.

The different types of sensory receptors an animal can have include photoreceptors (light), chemoreceptors (taste and olfaction), mechanoreceptors (touch, perception, balance, and hearing), electroreceptors (electrical fields) and thermoreceptors (temperature changes). These sensory receptors are responsible for sending a signal to the brain, which then integrates this information and sends out a signal to an effector organ. An effector organ is responsible for the output process, which produces the response by having these receptors become depolarized, thus creating an action potential.

In different animals the sensory receptors present, the senses animals are able to detect and their complexity changes. In order to illustrate this, let’s briefly compare the olfactory systems of vertebrates and invertebrates.

Vertebrates:

Have a more complex olfactory system allowing them to detect 1000’s of smells

This system is present in the roof of the nasal cavity and contains an olfactory epithelium, which is moistened by mucous

Odorant receptors are positioned within this olfactory epithelium and attach to odorant binding proteins

Each one of these proteins represents one scent

Invertebrates:

The olfactory system is usually near the head, but can be anywhere in the body

Insects have a model system, in which this system is on their antennae

This system is composed of a cuticle with hair-like projections (called sensilla) which have odorant receptor neurons